1. Field of the Invention
The present invention relates to an image forming apparatus employing an electrophotographic method.
2. Description of the Related Art
An electrophotographic image forming apparatus includes a photosensitive drum, i.e., an image bearing member bearing a toner image, and an intermediate transfer belt (ITB), i.e., an intermediate transfer member. Further, it is necessary for such an image forming apparatus to drive the photosensitive drum and the ITB so that surface speeds thereof are constant.
More specifically, it is necessary for the surface speeds to be constant for the following reasons. Laser exposure which is performed for forming an electrostatic latent image on the photosensitive drum may be time synchronization exposure. In such a case, a change in the surface speed of the photosensitive drum causes a laser irradiation position to be displaced from the intended position.
Further, in a primary transfer process, a toner image formed on the photosensitive drum is transferred to the ITB. In such a case, if there is a periodic speed difference between the surface speeds of the photosensitive drum and the ITB, the position of the toner image transferred to the ITB becomes displaced from the intended transfer position.
As a result, image failure occurs with respect to an image output to recording paper, such as color misregistration between the colors, and banding, i.e., a periodic positional displacement.
To solve such a problem, various speed detection sensors are used in performing drive control of the photosensitive drum and the ITB. Speed feedback control is thus performed on motors driving the photosensitive drum and the ITB to realize a highly accurate constant speed. A brushless direct current (DC) motor (BLDC motor) is often used as the motor due to low cost, quietness, and high efficiency thereof.
In recent years, speed feedback control using the BLDC motor has been performed as follows. A rotary encoder is arranged on a drum shaft, and motor control is performed to control a rotation speed of the drum shaft to be constant.
In the above-described method of speed feedback control, the rotation speed of the drum shaft is detected. However, since the surface speed of the photosensitive drum is not detected, it is difficult to control the surface speed of the photosensitive drum to be constant due to eccentricity of the drum shaft, and accuracy of a drum diameter.
Further, it is similarly difficult to control the surface speed of the ITB to be constant due to eccentricity of a shaft of an ITB drive roller 110 (illustrated in FIG. 1) which drives the ITB, the accuracy of a roller diameter, and unevenness in the thickness of the ITB.
Furthermore, mutual interference due to friction between the photosensitive drum and a transfer surface of the ITB may cause the image failure. More specifically, speed fluctuation in one of the photosensitive drum and the ITB is transferred to the other, and thus causes such a mutual interference.
Moreover, if the toner image formed on the ITB is to be transferred to a cardboard in a secondary transfer process, a sudden change in a load on the ITB occurs so that high-frequency speed fluctuation is generated. The high-frequency speed fluctuation then causes the positional displacement in the secondary transfer.
As described above, there are various causes of the image failure, and it is extremely difficult to solve all of the problems.
Japanese Patent Application Laid-Open No. 2002-333752 discusses such a configuration that an image bearing member corresponding to the photosensitive drum is driven with friction by an image transfer member corresponding to the ITB. In this configuration, an image is formed based on a position reference of the photosensitive drum for the image developed on the photosensitive drum to become an image on the ITB. An effect of unevenness in the rotation of the photosensitive drum can thus be reduced.
Further, according to the technique discussed in Japanese Patent Application Laid-Open No. 2002-333752, the image developed on the photosensitive drum becomes the image on the ITB even if there is the speed fluctuation of the ITB due to an impact on the ITB when the recording paper enters a secondary transfer unit. As a result, the image failure does not occur in the primary transfer.
However, as discussed in Japanese Patent Application Laid-Open No. 2002-333752, it is necessary to increase transfer pressure in the primary transfer to realize the driven operation of the photosensitive drum with friction. If the transfer pressure is increased, the load generated on the photosensitive drum and the ITB increases, so that an amount of torque in the motor increases and thus has an adverse effect on transferability of toner.
On the other hand, if the driven operation is realized without increasing the transfer pressure, a difference between the rotation speeds of the photosensitive drum and the ITB is generated with time due to a load change on the photosensitive drum. The driven operation thus cannot be continued.